1. Field of the Invention
The present invention relates to seals in gas turbine engines to restrict fluid flow between rotating and stationary engine members, and more particularly is related to a gas bearing face seal having improved sealing characteristics.
2. Description of the Known Art
Air leakage through gas turbine engine seals may significantly increase fuel consumption, reduce engine efficiency, and increase maintenance costs by increasing turbine inlet temperatures. Gas turbine engines have traditionally included labyrinth seals at critical sealing locations. Labyrinth seals control leakage of high pressure gas, such as compressor discharge air, from a generally high pressure area to a generally low pressure area. The seals operate by throttling gas flow through a series of annular constrictions formed between annular teeth, which may be located on a rotating component, and an annular rub strip, which may be located on a stationary engine member. The rub strips are abradable to allow the teeth to rub lightly during dynamic operating conditions, such as thermal transients or maneuver loads. The effectiveness of these labyrinth seals is dependent on keeping the radial clearance between the rub strip and teeth to a minimum. However, the minimum radial clearance is limited by manufacturing tolerances, rotor concentricity control, and thermal growth between rotating and stationary components. Too small a radial clearance results in premature seal wear and possible engine damage, while too large a radial clearance results in excess leakage. As seal diameters increase and gas temperatures increase in advanced engines, the radial clearance must increase, thereby lowering the effectiveness of the labyrinth seals.
Gas bearing face seals provide an alternative to the labyrinth seal. U.S. Pat. No. 3,383,033, issued to C. Moore, and assigned to the assignee of the present invention, discloses a gas bearing face seal for use as a compressor discharge seal, and is incorporated herein in its entirety by reference. An air bearing is used to actively control the spacing between a flow restricting tooth and a rotating sealing surface on a rotating component. A face seal ring member, which carries the restricting tooth, is supported for movement toward the rotating sealing surface. A ring seal, such as a piston ring seal, provides a secondary seal between the ring member and a stationary engine frame.
During low or no power conditions the ring member and restricting tooth are biased away from the rotating sealing surface by springs. During higher power operation high pressure compressor discharge air acts on the ring to urge the ring and tooth toward the sealing surface. A portion of the high pressure discharge air is supplied to a gas bearing space between the ring and the rotating sealing surface to establish a predetermined gas bearing face clearance. Pressure forces developed in the gas bearing space oppose further motion of the ring and tooth toward the sealing surface, and permits close spacing of the restricting tooth with respect to the sealing surface by actively maintaining the predetermined clearance. Further motion of the ring and tooth toward the rotating sealing surface increases the pressure forces in the air bearing space, thereby urging the ring and tooth away from the sealing surface to maintain the predetermined clearance.
While the seal as disclosed in U.S. Pat. No. 3,383,033 attempts to overcome disadvantages of the labyrinth seal, the disclosed seal, itself, includes a number of significant disadvantages. First, the disclosed seal does not include means for maintaining the ring member concentric with respect to the axis of the engine or with other seal components. The applicant has found that concentricity of the ring member with respect to other seal components is important for proper seal operation. Concentricity helps to maintain concentric, balanced pressure forces on the seal components. Concentric, balanced pressure forces, in turn, promote good sealing characteristics while permitting the use of smaller, more lightweight seal components. In addition, pressure forces that are not concentric with respect to the engine centerline can result in nonconcentric forces acting on critical rotor assembly components.
Second, the seal as disclosed shows the ring member pressurized radially inwardly by the higher pressure region. Rings pressurized radially inwardly will deform to an out-of-round shape with reduced sealing capability unless they are sufficiently massive and stiff. However, the ring member should ideally have a low mass inertia and relatively high compliance for reduced seal weight and, more importantly, reduced hysteresis in the clearance between the radial sealing surface and the restricting tooth. A low mass inertia ring can more quickly and efficiently track motion of the sealing surface with lower actuating forces.
Third, differential thermal growth and other effects influencing the clearance between the seal housing and face seal ring member can result in changes in pressure forces acting on seal components, which can result in poor sealing.
Fourth, the seal as disclosed includes an auxiliary restrictor tooth integral with or mounted on the ring member, which adds weight to the ring member, and increases pressure closing forces on the ring member, with the result that heavy spring means must be used to bias the ring member away from the sealing surface.
Fifth, the seal as disclosed does not efficiently vent air exiting the air bearing space and the restrictor tooth to the low pressure region. Improved venting is desired to assure proper pressure balance on the ring member.